The invention relates to a driver circuit of a liquid crystal display, and more particularly to a shift register circuit reducing the voltage shifting.
In the majority technology of LCDs a driver circuit is disposed on the glass substrate thereof, and the reduction of the cost of the driver IC is advantageous. Take the TFT LCD for example, instable features, such as voltage shifting of the threshold voltage, has become a critical issue of the circuit design. Please refer to FIG. 1. FIG. 1 is a voltage-current schematic diagram of a TFT transistor fabricated in a 300um process. Curves 11, 12, 13, 14 and 15 respectively indicate the voltage-current curve of the TFT transistor after 0, 2, 4, 6 and 8 continuous hours of using. It is found that the shifting of the threshold voltage is affected by the hours of use. The shifting of threshold voltage causes the shift register to output an incorrect signal, thus, the display cannot show the correct output.
Please refer to FIG. 2. FIG. 2 is a circuit diagram of a conventional shift register. Transistors T21 and T22 turn on in response to receiving voltage VDD, and thus, the threshold voltage shifts and the output signal N cannot stay in the off state. Please refer to FIG. 3. FIG. 3 is a schematic diagram of the output signal of shift register of FIG. 2. Curve 31 is the voltage-time curve when the shift register of FIG. 2 begins operating. Curve 32 is the voltage-time curve of the shift register of FIG. 2 after 6 continuous hours of use. According to curve 32, the conventional shift register cannot keep the output in the off state, and thus the display cannot output the correct signals.